1. Field of the Invention
The present invention relates generally to a communication system, and in particular, to an apparatus and method for allocating sub-channels taking moving velocities of mobile stations into account in a wireless communication system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA), both of which are a data transmission scheme using multiple carriers, are a type of Multi-Carrier Modulation (MCM) scheme that converts a serial input symbol stream into parallel symbol streams, and modulates them with multiple orthogonal sub-carriers, i.e., multiple orthogonal sub-carrier channels, before transmission.
In the wireless communication system that transmits data using multiple carriers, sub-carriers in one OFDM symbol are shared by a plurality of users. The wireless communication system uses a plurality of sub-carriers. Of the sub-carriers, some sub-carriers are used as pilot sub-carriers, and the other sub-carriers except for the pilot sub-carriers are used as data sub-carriers. The wireless communication system classifies the data sub-carriers into sub-channels each having a predetermined number of data sub-carriers, and allocates the sub-channels to a plurality of users according to system conditions.
Herein, the term “sub-channel” refers to a channel that includes a plurality of sub-carriers. As a result, the wireless communication system aims at acquiring frequency diversity gain by spreading all sub-carriers, especially data sub-carriers used in the system over the full frequency band. With reference to FIG. 1, a description will now be made of a configuration of a wireless communication system using multiple carriers (known as a multi-carrier wireless communication system).
FIG. 1 is a diagram schematically illustrating a configuration of a general wireless communication system.
For the sake of clarity, it will be assumed in FIG. 1 that the wireless communication system has a single-cell configuration, and includes a base station (BS) 100 and mobile stations (MSs) 110 and 120 managed by the BS 100.
The channel environments measured by the MSs 110 and 120 suffer different shadowing losses and path losses, are independent of each other according to movements of the MSs, and have time-varying characteristics. Due to these channel characteristics, the BS 100 allocates sub-channels to the MSs taking the channel environments into account, thereby increasing the overall data transmission efficiency of the system. The BS 100 employs an Adaptive Modulation and Coding (AMC) technique that adaptively changes a modulation technique and a coding technique taking into account the channel environments of the MSs 110 and 120. That is, the BS 100 uses the high-efficiency modulation and coding techniques for a good channel environment, and the low-efficiency modulation and coding techniques for a bad channel environment.
The BS 100 sends a Channel Quality Information (CQI) request to the MSs 110 and 120 located in its service coverage to allocate sub-channels to the MSs 110 and 120. The MSs 110 and 120 measure CQI of each sub-channel in response to the CQI request from the BS 100.
The MSs 110 and 120 feed back, i.e., transmit, their respective measured CQI to the BS 100. The MSs 110 and 120 periodically transmit the CQI to the BS 100. The BS 100, upon receiving the corresponding CQI, performs sub-channel allocation using the corresponding received CQI.
In this wireless communication system, a BS transmits a control signal to corresponding MSs to perform resource allocation, i.e., sub-channel allocation, to the corresponding MSs. The control signal includes sub-channel allocation information and Modulation and Coding Scheme (MCS) level information for the sub-channel allocation. However, the BS allocates sub-channels to the MSs in a one-frame interval, i.e., every frame. As the control signal for sub-channel allocation is transmitted to MSs in the cell every frame, the control signal for sub-channel allocation may serve as signaling overhead when data is transmitted to MSs in a communication system.